Known rotary type carburetors are commonly used for small, two stroke combustion engines applied to applications such as handheld chainsaws, leaf blowers, hedge trimmers, and the like. A rotary throttle valve of the carburetor has a generally cylindrical throttle received rotatably in a cylindrical cavity of a body that intersects a fuel-and-air mixing passage extending through the body. The rotary throttle valve rotates about an axis and, via a cam interface between the body and the cylindrical throttle, is also operatively moveable axially within the cylindrical cavity. The axial position of the cylindrical throttle is thus dependent upon the rotational angular placement of the throttle valve between a closed or idle position and a wide open throttle, WOT, position. A through-bore extends transversely through the cylindrical throttle of the rotary throttle valve and variably aligns generally longitudinally with the fuel-and-air mixing passage. The through-bore extends substantially perpendicular to the rotary axis and aligns so that when the carburetor is in the WOT position the through-bore is in substantially full communication with the fuel-and-air mixing passage, and when the through-bore is substantially misaligned or communicating minimally with the mixing passage the rotary throttle valve is either closed or in the idle position.
A fuel feed tube disposed concentrically to the rotary axis projects upward from the body and into the through-bore of the cylindrical throttle to an open distal end. An idle adjustment needle is fixedly attached to and projects downward from the cylindrical throttle, into the through-bore, and through the open distal end of the fuel feed tube. An orifice or jet for flowing liquid fuel into the through-bore communicates through a wall of the fuel feed tube and variably aligns axially with the idle fuel adjustment needle in the tube thereby varying the flow cross section of the orifice. Because the rotating cylindrical throttle is cammed, it moves axially during rotation carrying the idle fuel adjustment needle with it as the fuel feed tube and orifice remain stationary. The flow cross section of the orifice generally increases as the throttle valve moves toward the WOT position.
The idle fuel adjustment needle is threaded to the cylindrical throttle enabling adjustment of the quantity of liquid fuel entering the through-bore generally at idle operating conditions of the engine. When the idle fuel adjustment needle is threaded further into the cylindrical throttle more of the orifice is obstructed thus the flow of liquid fuel through the orifice and into the through-bore is reduced. During engine high speed operation or WOT, the idle fuel adjustment needle has little to no effect on fuel flow through the orifice. Instead, a separate high speed fuel adjustment needle is utilized. The high speed fuel adjustment needle threads into the body of the carburetor to adjustably obstruct a primary fuel feed passage in the body that flows liquid fuel to the fuel feed tube. By threading the high speed fuel adjustment needle further into the body, more of the passage is obstructed and less fuel flows through the orifice of the fuel feed tube at high engine speeds.
During manufacturing assembly of the rotary carburetor, and presuming the valve axis is vertical for purposes of explanation, the cylindrical cavity is typically opened upward for receipt of the rotary throttle and is aligned and supported axially between a bottom of the cylindrical cavity carried by the body and a top cover fastened to the body. A hollow shaft of the rotary throttle disposed concentrically to the rotary axis projects rigidly from the throttle and through the cover to engage a throttle lever having a hole for access to the idle fuel adjustment needle located inside the shaft. Unfortunately, access to the high speed adjustment needle must be provided on the side of the carburetor body and not the top like the idle adjustment needle because the top is generally congested with the cover, throttle lever and cam mechanism of the carburetor. Moreover, the fuel passage leading to the fuel nozzle is located centrally in the carburetor main body, and the high speed adjustment needle must extend into the fuel passage in an axially moveable manner with respect to the passage.
Therefore, when conducting the idle and high speed calibration and operation tests of the carburetor in the assembly line or at the time of servicing, the adjustment must be conducted from two different directions, and this inconveniently complicates the adjustment work. Furthermore, when the carburetor is mounted on the engine, it is necessary to provide access for adjustment of the carburetor from two different directions and locations. This restricts the layout of other auxiliary equipment and component parts.